Last month Microsoft said that it was considering ending support for TLS and SSL certificates that used the SHA-1 hashing algorithm, after Mozilla previously described a plan to do the same. Google is now thinking about joining those two companies and ending Chrome's support for SHA-1 certificates in the middle of next year too.
The underlying problem is that it has become too cost-effective to create forged certificates that use the SHA-1 hashing algorithm. As computers get faster, the cost of creating a fraudulent certificate goes down. Based on 2012 estimates, it was expected that criminals would be able to readily create such certificates by 2018. This declining cost led all three browser vendors to plan to end supporting any SHA-1 certificates issued after January 1, 2016, and all SHA-1 certificates after January 1, 2017.
Newer estimates have brought the cost of certificate fraud down further still. Through the use of cloud services such as Amazon's EC2, the compute power to create bogus SHA-1 certificates both costs less and is more accessible, such that SHA-1 certificates are arguably unsafe already. This led to reconsideration of the 2017 timetable. Mozilla and Microsoft are now contemplating bringing that January 1, 2017 date forward, to July 1, 2016, as long as the impact in-the-wild is not too serious.